couchbase-cli: Reduce type flipping

In a number of places in cbmgr.py "type" of a object is converted:

       if opts.enabled == "1":

            opts.enabled = "true"

        elif opts.enabled == "0":

            opts.enabled = "false"

It should be possible to get arg_parse to do this via a help function:

https://stackoverflow.com/questions/15008758/parsing-boolean-values-with-argparse